What happened in the last one?

The animal rights activist from the first episode came back and tried making them live in the wild. After a while, they tell him that what he is doing is animal abuse and convinces him to take them back to the lab.

There, its revealed that he is such an extremist because he belived he had killed his pet rabbit as a child because he left a bucket of water with a dead fly in it. He believed that the rabbit drank the water, and the dead fly's urine killed it. The animals explain that even if a fly did pee, there's no way such a small amount could have killed the rabbit.

The animals head back to their room, where they discover the scientists have replaced them with other animals (except for the bird). Furthermore, all the replacements have accomplished more (the horse has published a book, the dog has a child, the rat has bigger boobs). Suddenly, the activist goes mad believing his life to be sham. He starts killing all the lab animals, so the four run back to their house.